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In high altitude populations of yellow-billed pintail, hemoglobin has a higher affinity for oxygen than in lower altitude populations, which can be attributable to substitutions in their beta-globin gene. These substitutions are shared by speckled teal because of introgressive hybridization between the two species. Gene flow between populations also suggests that yellow-billed pintails that are heterozygous for the βA hemoglobin subunit may be able to acclimate to high altitudes more efficiently than those that are homozygous for the βA hemoglobin subunit.